Example: Sherds from Mari, Syria and Lassy, France
Above: Ceramic sherds from Tell Haririr (Mari) in Syria (left) and a medieval site in Lassy, France (right)
I was brought the two sherds pictured above awhile ago. The sherd on the left is circa 1850-1650 BCE and came from Tell Hariri, the ancient city of Mari, on the banks of the Euphrates River in Syria, approximately 120 km southeast of Deir ez-Zor. Mari was at its height between about 2900 BCE and 1760 BCE, when the Babylonian king Hammurabi sacked the city, so the sherd came from about that time. The second sherd, on the right, is circa 1000-920 CE came from a very different site: a medieval site in Lassy commune of northwestern France.
These two sherds from different times and places each have a different remnant magnetism. When the clay was fired to make the original pots, tiny iron particles in the clay aligned their magnetic domains with Earth's magnetic field, and these domains retained this orientation after the clay cooled. Earth's magnetic field changes in its orientation and strength over time, and this phenomenon is of interest to both geologists and archaeologists. Paleomagnetism can be used for dating clay artifacts, making it useful in archaeology. It is also interesting to geologists, who use the remnant magnetism of artifacts as well as rocks and sediment to investigate polar wandering, plate tectonics, the fluid dynamics of Earth's core, and other topics in geophysics. This is the research focus of the Institute for Rock Magnetism (IRM) in the Geology and Geophysics Department here at the University of Minnesota.
That is how these sherds came to me: researchers in the IRM were trying to measure the remnant magnetism of the two sherds and were hoping to use the data for their studies. There was a problem, though. Samples taken from different parts of the sherds were yielding significantly different results. This can indeed be a problem when analyzing ceramics because they are mixtures, that is, they are made up of distinct ingredients: a clay matrix, naturally occurring inclusions within the clay, and deliberately added temper. Ceramics are also usually layered materials: they can have slips, glazes, paint, and surface alteration due to firing and/or weathering.
I was asked to help them sort out what was causing different from different parts of the sherds. A possible cause was so-called "chemical remnant magnetization," that is, when chemical alteration of iron-rich minerals has caused the magnetic domains to realign at the time of the chemical change. Such alteration could occur during weathering and/or firing. Another possibility was that, when the ceramics were fired, the interiors did not reach the Curie point. The Curie point is the temperature at which a material's magnetic domains are disrupted by thermal fluctuations and become free to realign with Earth's magnetic field. If a ceramic's interior did not reach the Curie point, its interior magnetic domains may have retained their original orientations. A third possibility, related to the first, was that organics, which can cause reducing conditions, were burned away in the exterior during firing and less so on the interior. Other possibilities were considered, most related to surface alteration or distributions of iron-rich particles.
The remnant magnetism in clay is due primarily to iron oxides, magnetite and hematite. Other minerals, including titanomagnetites (titanium-iron oxides), siderite (iron carbonate), goethites (hydrous iron oxide), and amphiboles (iron- bearing silicates), can contribute as well. Usually these components make up less than 1% of the sherd. Distributions of such minerals were the main focus of my investigations. This was a question well suited to the electron microprobe, where backscattered-electron images can readily reveal heavier iron-rich minerals within the lighter clay matrix and the compositions of those minerals can be established using the X-ray spectrometers.
So what was the answer? I don't think that I can share the results publicly yet... Maybe later.
5/30/07
Added:
Electron Microprobe Analysis in Archaeology
Electron microprobe analysis (EMPA), also known as electron probe microanalysis (EPMA), is an analytical technique that combines scanning electron microscopy (SEM) and compositional analysis using x-ray spectrometry. The ability to determine structure and chemistry of samples makes EMPA very versatile. This is a dominant analytical technique in geology, but it is not as commonly used in archaeology despite similar materials in studied both fields. Here I will post about topics in EMPA, artifacts I have analyzed, archaeological studies that use EMPA, etc. If there is a topic you'd like to see posted here, please let me know.
Ellery Frahm
Doctoral Candidate, Archaeology
Research Fellow, Geology & Geophysics
University of Minnesota - Twin Cities
